1. Field of the Invention
The present invention relates to a radio communication system having a plurality of base stations respectively connected over a radio channel to a plurality of personal stations in a time division multiple access manner employing a communication frame having a plurality of time slots, and more particularly it relates to a radio communication system in which a plurality of base stations are situated neighboring one another and which alleviates the disadvantages encountered when radio wave interference occurs between the base stations.
2. Description of the Related Art
One example of a known configuration of digital mobile communication system is a radio communication system in which, as shown in FIG. 6, base stations BS1 and BS2 are respectively connected via an ISDN line and a two-wire time division transmission line to a private branch exchange PBX1 which is connected to an ISDN 100, and in which BS1 and BS2 are connected to personal stations PS1 and PS2 over a radio channel.
The transmission and reception timing in this radio communication system is decided by the timing of the slots (time slots) and frames (communication frames) of the base stations. For example, the transmission timing of a first absolute transmission slot in a communication frame of a base station such as BS1 in the figure, which is connected to the private branch exchange PBX1 via the ISDN line, is decided by the arrival timing of a signal through a circuit.
In contrast, with a base station such as BS2 shown in the same figure, which is connected to the private branch exchange PBX1 by the two-wire time division transmission line, the arrival timing of the circuit signal does not necessarily match the ISDN line. Therefore, at present, when a device is constructed as shown in FIG. 6 so that the neighboring base stations BS1 and BS2 are respectively connected to a private branch exchange PBX1 via an ISDN line and two-wire time division transmission line, the slot and frame timings of the base stations are basically asynchronous.
Now, in general, base stations in this type of conventional radio communication system are configured as shown in FIG. 7 for example, and are configured so that, when communication is to take place between such a base station and a corresponding personal station, the reception field strength of each slot is detected based on reception output from the radio unit 21, an idle slot is detected based on this reception field strength, and this idle slot is determined to be the communication slot and the communication is thereby able to take place.
In other words, the communication slot for a base station in a conventional device of this type is determined by a routine such as that shown in FIG. 8. More specifically, the arrangement is such that the frame and slot timing is determined in advance by the specific timing of the base station (Step 801), when there is a communication request (Yes in Step 802) an interference detection unit 22 senses the reception field strengths of the slots fixed by this timing (Step 803), and if there is an idle slot (Yes in step 804) this is determined to be the communication slot (Step 805), the system moves into communication mode (Step 806), and once the communication has finished (Yes in Step 807) it returns to Step 802 ready for the next communication request.
Now, given a conventional device (see FIG. 6) employing base stations 2 of the form shown in FIG. 7, consider the situation where the personal station PS1 and the base station BS1 are communicating with a second slot using a frequency f1 and then the base station BS2 and the personal station PS2 try to start communicating on a frequency f2 which is close to the frequency f1.
In such a situation, given a conventional device in which the base stations BS1 and BS2 set slots and frames using their own timings and the slot and frame timings are set without taking neighboring base stations into account, it can sometimes happen that the slot and frame timing of the base station BS2, which tries to start communication later, assumes a timing as shown in FIG. 9(b) with respect to the slot and frame timing of the base station BS1 which is already in communication as shown in FIG. 9(a).
More specifically, FIGS. 9(a) and 9(b) show a state in which there are two slots 2 and 3 for the base station BS2 corresponding in time to the slot 2 for the base station BS1, and in this case the interference level detection unit 22 of the base station BS2 judges that neither slot 2 nor slot 3 is an idle slot, which means that the two slots, that is slot 2 and slot 3, cannot be used. As a result, the slot usage efficiency is markedly reduced.
Thus, since the abovementioned conventional radio communication system is so configured that slots/frames are not synchronized between base stations connected to a private branch exchange PBX by separate circuits, it has a problem in that when a base station which is about to start communication detects radio wave interference with a neighboring base station which is already in communication, it can happen that two slots are unable to be used in a timing of the slots/frames of that base station and the slot usage efficiency is markedly reduced.